1. Field of Invention
The present invention generally relates to a non-volatile memory and, particularly to a non-volatile memory formed with a SONOS structure.
2. Description of the Related Art
A non-volatile memory has the advantages of the ability to perform multiple times of write, read or erase operation for data, and the stored data would not disappear after power down. The conventional non-volatile memory primarily uses polysilicon material as a charge storage layer thereof.
Owing to the characteristic of silicon nitride trapping electrons, in the conventional non-volatile memory technology, the silicon nitride is popularly used as the charge trap layer to replace a polysilicon floating gate. Generally, the silicon nitride is formed with two silicon oxides respectively disposed above and below the silicon nitride to serve as a charge tunnel layer and a charge block layer, and thereby forming a composite layer of oxide-nitride-oxide (ONO). Such type of non-volatile memory generally is termed as polysilicon/silicon oxide/silicon nitride/silicon oxide/silicon (SONOS) memory device.
According to different configuration locations of the silicon nitride charge trap layer, the SONOS memory devices can be classified into planar-type and sidewall-type. The silicon nitride charge trap layer of the planar-type SONOS memory device is disposed between two silicon oxide layers, the two silicon oxide layers are formed with two silicon layers disposed above and below the two silicon oxide layers and respectively serving as a polysilicon gate (control gate) and a silicon substrate. However, the planar-type SONOS memory device uses the ONO structure to replace the gate oxide layer and thus is incompatible with the current logic processes, and would further increase the process complexity so that the efficiency of the memory device is negatively influenced.
In addition, the silicon nitride charge trap layer of the sidewall-type SONOS memory device is disposed on the sidewall of the gate, or disposed on the sidewall of the gate and a part of the silicon substrate. Therefore, during the operation of the sidewall type SONOS memory device, electrons are necessarily required to be injected into the silicon nitride charge trap layer, which would easily encounter the issue of electron drift and influence the operation speed and charge storage capability of device.